Pressurized water nuclear reactors with spectral shift control which have been described in U.S. patent application Ser. No. 531,786 filed Sept. 13, 1983 and U.S. Pat. No. 4,544,521 are known.
Such reactors permit a better use of the nuclear fuel by modifying the neutron energy spectrum in the course of the successive phases of the operating cycle of the reactor. During the first part of the operating cycle of the reactor, clusters of elements of neutron-absorbing material are introduced into the core to produce a hardening of the neutron spectrum, both by reducing the volume of moderator in the core and by absorbing low energy neutrons. In a second part of the operating cycle of the reactor, the clusters of absorber elements are withdrawn from the core of the reactor and the fissile material formed during the first phase is consumed. The elements of absorber material are preferably of a fertile material capable of being converted into fissile material under the effect of the neutron bombardment. This fertile material is most frequently uranium constraining a low proportion of uranium 235.
Nuclear reactors with spectral shift control are controlled, so far as their power is concerned, in the same was as conventional pressurized water nuclear reactors. This control of power is ensured by an assembly of control rods, each consisting of absorber clusters which are moved vertically inside the reactor core by means of driving mechanisms which usually consist of electromagnetically controlled ratchets. The core of the reactor consists of prismatic assemblies arranged vertically and side by side, each comprising an assembly of guide tubes into which can be introduced at a variable depth the absorber elements of the cluster forming the control rod. The control rods are associated only with a part of the reactor core assemblies and each of these assemblies receiving a control rod is associated with a mechanism for precise vertical movement of this control rod.
In contrast to the control rods, the absorber clusters for spectral shift control remain in a fixed position in the assemblies which receive them in the course of a phase of the operating cycle of the reactor, i.e., in a position of maximum insertion during the first phase of the cycle and in a position of complete withdrawal during the second phase of the cycle.
U.S. Pat. No. 4,544,521 discloses a control device which makes it possible to carry out both the movement of a control rod for the control of reactor power and the withdrawal of an absorber cluster for spectral shift control associated with the same assembly, at the end of the first phase of the operating cycle of the reactor.
In fact, it is preferable to associate the control rods and the clusters for spectral shift control with the same fuel assemblies which comprise a first assembly of guide tubes intended to receive the elements of the control rod and a second assembly of guide tubes intended to receive the absorber cluster for spectral shift control.
U.S. Pat. No. 4,544,521 describes a device comprising a first tubular control shaft to the lower part of which is fixed the control rod and a second control shaft arranged coaxially with the first inside the latter, to the lower part of which is fixed the absorber cluster for spectral shift control.
The first control shaft comprises, on its outer surface, grooves allowing it to be moved stepwise by a conventional ratchet device. This control shaft moves inside a vertical leaktight enclosure of a great height, fixed to the cover of the vessel and communicating with the inner volume of this vessel. The ratchet mechanisms are fixed to this enclosure. At the end of the upward movement, the first control shaft can move a few additional steps beyond its position corresponding to the maximum withdrawal of the control rod. This high overrun of the first control shaft allows the fingers for hooking the second control shaft to the first to be placed in an open position.
The second control shaft comprises a widened part equipped with sealing segments forming a piston inside the first control shaft whose inner surface forms the corresponding cylinder. The sealing segments associated with the piston are therefore of an outer diameter which is identical to the inner diameter of the first control shaft so as to produce a leaktight seal between the piston and the cylinder. An exhaust valve is arranged in the upper part of the leakproof enclosure, the opening of this valve producing a reduction in pressure in the upper part of the inner volume of the first control shaft above the piston of the second control shaft. The opening of this valve, at the end of the first phase of the operating cycle of the reactor, the first control shaft being in a high overrun position, makes it possible to withdraw the cluster of absorber elements for spectral shift control and to place it in a high position inside the first control shaft. In this position it is possible to effect the hooking of the second control shaft on the first by virtue of the pivoting fingers carried by the first control shaft.
During the second phase of the operating cycle of the reactor, the two control shafts are united and the absorber cluster for spectral shift control follows the movements of the control rod. In this second phase of the cycle, however, the control rods remain practically always in a withdrawn position.
Such a device, which is very simple and which does not require structural modifications of the reactor, presents nevertheless the disadvantage of retaining a sliding contact between the sealing segments of the piston formed by the second control shaft and the interior surface of the first control shaft at every movement of the control rod during the first phase of the operating cycle of the reactor. The absorber cluster for spectral shift control and the second control shaft are, in fact, stationary during this entire phase while the first shaft and the control rod are moved very frequently, mainly in a localized zone.
Although the piston of the second control shaft is used solely at the end of the first phase and for an extremely short period, the sealing segments of this piston risk being worn and deteriorated by the rubbing on the inner surface of the first control shaft.
Furthermore, during the upward movements of the first control shaft, the second shaft runs the risk of being dragged along since the sealing segments are in constant contact with the interior surface of this first control shaft.